We use the instant individual fitness (wellbeing) \(W\)  as a characteristic of the current state of the organism, reflecting the non-optimality of its performance: \(W=1\)  means the maximal achievable performance, \(W=0\)  means inviability (death). If the organism lives at some level of \(W\)  then we can consider \(W\)  as a factor in the lifetime fitness. Such a factorization assumes that the physiological state of the organism acts independently of other factors to determine fitness. This assumption follows the ideas of [1]}. The basic assumptions of Fisher's model were analysed by [2]}. `Independence' here is considered as multiplicativity, like in probability theory. Of course, the hypothesis of independence is never absolutely correct, but it gives a good initial approximation in many areas, from data mining (naïve Bayes models) to statistical physics (non-correlated states).
